Atmospheric Pressure Ionization is used in analytical instruments, such as Atmospheric Pressure Chemical Ionization (APCI) Mass Spectrometry, and in detection devices. The detection devices can be either conventional Ion Mobility Spectrometers (IMS) or Differential Mobility Spectrometers (FAIMS). These devices typically use a small radioactive source that generates energetic particles (electrons, alpha particles) that when introduced into the surrounding gas ionize some atoms or molecules.
There have been attempts to replace the radioactive source commonly used in APCI. There are corona discharges, uv-ionization, laser induced ionization, and other plasma discharges. However, these sources have drawbacks and lack the flexibility of an ionization source which launches energetic particles into the gas.
Electron beams are used commercially for treatment of surfaces and gases. These electron beam units have a large evacuated volume, with a thermionic cathode, usually at high voltage facing a thin anode-window at ground, that allows transmission of the beam, usually in a triode configuration. The current is adjusted by either appropriate heating of the cathode, or through appropriate biasing of the controlling the intermediate voltage, while the electron energy is determined by the voltage drop from the cathode to the anode. These units required a continuous vacuum in order to prevent breakdown that could destroy the thin window. In addition, the high voltage at the cathode requires high-voltage feedthroughs that are large.
Compact electron beams have been contemplated as an ionization source for APCI. In the patent literature, Vitaly Budovich (U.S. Pat. No. 5,969,349, October 1999) teaches the use of an electron beam as the ionization source. The source has a window, preferably mica, and an evacuated volume with a hot cathode or a photocathode. Hans-Rudiger Donzig (U.S. Pat. No. 6,429,426, August 2002) teaches the use of an electron beam source used to make x-rays. In this case, the electrons do not have to be extracted from the evacuated volume. More recently, Hans-Rudiger Donzig (U.S. Pat. No. 6,586,729, July 2003), teaches the use of a current controlled e-beam for the control of x-ray emission, using a sustainer (in a triode configuration). This patent also teaches a scheme for monitoring the pressure in the tube and evacuating the tube when the pressure is too high.
These patents, and in particular U.S. Pat. No. 5,969,349, teach an electron source with a cathode with a high voltage feedthrough. This high potential is needed for acceleration of the electrons using conventional acceleration technology. However, conventional technologies present issues with the size of DC power supplies (including the transformer), the size of the high voltage insulators and other issues dealing with high voltage such as arcing. Alternatives to the high voltage requirement for high energy electron beams could result in significantly more robust and compact devices.
U.S. Pat. No. 7,105,808, Plasma ion mobility spectrometer, describes a compact electron beams that does not require a high voltage feedthrough, but still requires active voltage control. The electron beams concepts in this patent use an internal step-up transformer or a microwave electromagnetic fields in a cavity, the electromagnetic field matching the electron cyclotron resonance an externally imposed steady magnetic field. The electron current in these concepts is difficult to control and the devices are complex and large.
The use of pyroelectric crystals has been suggested for the generation of electron and ion beams. See, for example, Generation of focused electron beam and X-rays by the doped LiNbO3 crystal, M. Bayssie, J. D. Brownridge, N. Kukhtarev, T. Kukhtarev, J. C. Wang, Nuclear Instruments and Methods in Physics Research B 241 (2005) 913-916, and Electron and positive ion acceleration with pyroelectric crystals, Jeffrey A. Geuther and Yaron Danon, Journal Of Applied Physics 97, 074109 (2005) However, it is difficult to control the voltage or the current of the device. Stable voltage and adjustable current would be useful in applications of electron beam ionization for APCI devices.
The present state of the art of pyroelectric electron beams lacks adequate current control, lacks continuous production, has relatively low currents, has problems with thermal conductivity which can result in crystal cracking, the window needs to be able to withstand high temperatures, discharge of the crystal needs to be minimized, thus making them unsuitable for applications for an Ion Mobility Spectrometer. In addition, means of achieving a sealed tube with long lifetime and with sufficient stability has not been proposed to date. It is the purpose of this invention to overcome these obstacles.